Specializing the algorithms for segmentation with classes
So far, we have been developing applications that split work into multiple independent jobs and created classes to generalize the algorithms for segmentation. We simplified the creation of segmented and parallelized algorithms, generalizing behaviors to simplify our code and to avoid repeating the same code on every new application. However, we did not do that using inheritance, a very powerful object-oriented capability that simplifies code re-use. C# is an object-oriented programming language that supports inheritance and offers many possibilities to specialize behaviors to simplify our code and to avoid some synchronization problems related to parallel programming. How can we use C# object-oriented capabilities to define specific segmented algorithms prepared for running each piece in an independent thread using ParallelAlgorithm and ParallelAlgorithmPiece as the base classes?
The answer is very simple—by using inheritance and the factory method class creational pattern (also known as virtual constructor). Thus, we can advance into creating a complete framework to simplify the algorithm optimization process. Again, we can combine multithreading with object-oriented capabilities to reduce our development time and avoid synchronization problems.
Besides, using classes to specialize the process of splitting a linear algorithm into many pieces will make it easier for the developers to focus on generating very independent parts that will work well in a multithreading environment, while avoiding side-effects.
Time for action – Preparing the parallel algorithm classes for the factory method
You made the necessary changes to the ParallelAlgorithmPiece and the ParallelAlgorithm classes to possibly find planets similar to Mars in the images corresponding to different galaxies.
NASA’s CIO was impressed with your parallel programming capabilities. Nevertheless, he is an object-oriented guru, and he gave you the advice to apply the factory method pattern to specialize the parallel algorithm classes in each new algorithm. That could make the code simpler, more re-usable, and easier to maintain.
He asked you to do so. The NASA scientists would then bring you another huge image processing challenge for your parallel programming capabilities—a sunspot analyzer. If you resolve this problem using the factory method pattern or something like that, he will hire you! However, be careful, because you must avoid some synchronization problems!
First, we are going to create a new project with tailored versions of the ParallelAlgorithmPiece and ParallelAlgorithm classes. This way, later, we will be able to inherit from these classes and apply the factory method pattern to specialize in parallel algorithms:
- Create a new C# Project using the Windows Forms Application template in Visual Studio or Visual C# Express. Use SunspotsAnalyzer as the project’s name.
- Open the code for Program.cs.
- Replace the line [STAThread] with the following line (before the Main method declaration):
[MTAThread]
- Copy the file that contains the original code of the ParallelAlgorithmPiece and the ParallelAlgorithm classes (ParallelAlgorithm.cs) and include them in the project.
- Add the abstract keyword before the declarations of theParallelAlgorithmPiece and the ParallelAlgorithm classes, as shown in the following lines (we do not want to create instances directly from these abstract classes):
abstract class ParallelAlgorithmPiece
abstract class ParallelAlgorithm - Change the ThreadMethod method declaration in the ParallelAlgorithmPiece class (add the abstract keyword to force us to override it in subclasses):
public abstract void ThreadMethod(object poThreadParameter);
- Add the following public abstract method to create each parallel algorithm piece in the ParallelAlgorithm class (the key to the factory method pattern):
public abstract ParallelAlgorithmPiece
CreateParallelAlgorithmPiece(int priThreadNumber); - Add the following constructor with a parameter to the ParallelAlgorithmPiece class:
public ParallelAlgorithmPiece(int priThreadNumberToAssign)
{
priThreadNumber = priThreadNumberToAssign;
} - Copy the original code of the ParallelAlgorithmPiece class CreatePieces method and paste it in the ParallelAlgorithm class (we move it to allow creation of parallel algorithm pieces of different subclasses). Replace the lloPieces[i].priBegin and lloPieces[i].priEnd private variables’ access with their corresponding public properties access lloPieces[i].piBegin and lloPieces[i].piEnd.
- Change the new CreatePieces method declaration in the ParallelAlgorithm class (remove the static clause and add the virtual keyword to allow us to override it in subclasses and to access instance variables):
public virtual List<ParallelAlgorithmPiece>
CreatePieces(long priTotalElements, int priTotalParts) - Replace the line lloPieces[i] = new ParallelAlgorithmPiece(); in the CreatePieces method declaration in the ParallelAlgorithm class with the following line of code (now the creation is encapsulated in a method, and also, a great bug is corrected, which we will explain later):
lloPieces.Add(CreateParallelAlgorithmPiece(i));
- Comment the following line of code in the CreatePieces method in the ParallelAlgorithm class (now the new ParallelAlgorithmPiece constructor assigns the value to piThreadNumber):
//lloPieces[i].piThreadNumber = i;
- Replace the line prloPieces = ParallelAlgorithmPiece. CreatePieces(priTotalElements, priTotalParts); in the CreateThreads method declaration in the ParallelAlgorithm class with the following line of code (now the creation is done in the new CreatePieces method):
prloPieces = CreatePieces(priTotalElements, priTotalParts);
- Change the StartThreadsAsync method declaration in the ParallelAlgorithm class (add the virtual keyword to allow us to override it in subclasses):
public virtual void StartThreadsAsync()
- Change the CollectResults method declaration in the ParallelAlgorithm class (add the abstract keyword to force us to override it in subclasses):
public abstract void CollectResults();
What just happened?
The code required to create subclasses to implement algorithms, following a variation of the factory method class creational pattern, is now held in the ParallelAlgorithmPiece and ParallelAlgorithm classes.
Thus, when we create new classes that will inherit from these two classes, we can easily implement a parallel algorithm. We must just fill in the gaps and override some methods, and we can then focus on the algorithm problems instead of working hard on the splitting techniques.
We also solved some bugs related to the previous versions of these classes.
Using C# programming language’s excellent object-oriented capabilities, we can avoid many problems related to concurrency and simplify the development process using high-performance parallel algorithms. Nevertheless, we must master many object-oriented design patterns to help us in reducing the complexity added by multithreading and concurrency.
Defining the class to instantiate
One of the main problems that arise when generalizing an algorithm is that the generalized code needed to coordinate the parallel algorithm must create instances of the subclasses that represent the pieces.
Using the concepts introduced by the factory method class creational pattern, we solved this problem with great simplicity. We made the necessary changes to the ParallelAlgorithmPiece and ParallelAlgorithm classes to implement a variation of this design pattern.
First, we added a constructor to the ParallelAlgorithmPiece class with the thread or piece number as a parameter. The constructor assigns the received value to the priThreadNumber private variable, accessed by the piThreadNumber property:
public ParallelAlgorithmPiece(int priThreadNumberToAssign)
{
priThreadNumber = priThreadNumberToAssign;
}
The subclasses will be able to override this constructor to add any additional initialization code.
We had to move the CreatePieces method from the ParallelAlgorithmPiece class to the ParallelAlgorithm class. We did this because each ParallelAlgorithm subclass will know which ParallelAlgorithmPiece subclass to create for each piece representation. Thus, we also made the method virtual, to allow it to be overridden in subclasses. Besides, now it is an instance method and not a static one.
There was an intentional bug left in the previous CreatePieces method. As you must master lists and collections management in C# in order to master parallel programming, you should be able to detect and solve this little problem. The method assigned the capacity, but did not add elements to the list. Hence, we must use the add method using the result of the new CreateParallelAlgorithmPiece method.
lloPieces.Add(CreateParallelAlgorithmPiece(i));
The creation is now encapsulated in this method, which is virtual, and allows subclasses to override it. The original implementation is shown in the following lines:
public virtual ParallelAlgorithmPiece CreateParallelAlgorithmPiece
(int priThreadNumber)
{
return (new ParallelAlgorithmPiece(priThreadNumber));
}
It returns a new ParallelAlgorithmPiece instance, sending the thread or piece number as a parameter.
Overriding this method, we can return instances of any subclass of ParallelAlgorithmPiece. Thus, we let the ParallelAlgorithm subclasses decide which class to instantiate.
This is the principle of the factory method design pattern. It lets a class defer instantiation to subclasses. Hence, each new implementation of a parallel algorithm will have its new ParallelAlgorithm and ParallelAlgorithmPiece subclasses.
We made additional changes needed to keep conceptual integrity with this new approach for the two classes that define the behavior of a parallel algorithm that splits work into pieces using multithreading capabilities.
